A configuration of a conventional general NPC power converter is shown in FIG. 3. In the following description, a minimum on-pulse width control and a neutral-point potential fluctuation suppression control of the NPC power converter will be explained with reference to FIG. 3.
[Minimum On-Pulse Width Control]
In a case where a switching element (e.g. GTO) that has a limit on a minimum value of an on-pulse width (time) is used for switching elements SU1˜SU4, SV1˜SV4 and SW1˜SW4 of a three-phase NPC power converter 2, a pulse signal that is shorter than the limit of the minimum value of the on-pulse width might arise during a time period during which a voltage command value is present in the vicinity of zero-voltage. In such case, the switching element such as the GTO can not perform the switching then a voltage control becomes impossible for the time period for which the voltage command value is present in the vicinity of zero-voltage.
Further, also in a case where a switching element that has no limit of the on-pulse width is used in the three-phase NPC power converter 2, due to a dead-time that is set to prevent a short circuit of upper and lower arms of an inverter, an output pulse that is a dead-time width or less disappears, then the voltage control becomes impossible for that time period.
As a solution for the problems, there has been known a method in which a bias voltage is added to all phases of three-phase voltage command values so that all the phases of the three-phase voltage command values become a predetermined minimum on-pulse width or greater (see Patent Document 1). The bias voltages added here are set to the same value in all the three phases in order not to affect a line voltage of the three phases. Further, in general, as a method, this bias voltage is calculated so as to be a minimum in order to suppress fluctuation of the neutral-point potential due to this control.
[Neutral-Point Potential Fluctuation Suppression Control]
In a power conversion device 1 as shown in FIG. 3, when outputting a DC voltage of a neutral-point to the three-phase NPC power converter 2, a neutral-point current Io flows through a DC neutral-point. Therefore, due to this neutral-point current Io, voltages of two DC capacitor components at positive and negative sides which produce the neutral-point deviate from each other, then fluctuation occurs in a neutral-point potential Vo. It is generally known that the neutral-point potential Vo fluctuates at a cycle period of triple the output AC frequency.
As a method for solving this problem, a method, in which DC voltages VC1, VC2 of both positive and negative side capacitors C1, C2 are detected and the bias voltage calculated from their difference (e.g. calculated by a PID control etc.) is equally added to the three-phase voltage command values, has been known.
For instance, taking into consideration the fact that a direction of the bias voltage to be added changes between in a power-running state and in a regenerative state of drive mode of an inverter output, as shown in a neutral-point potential fluctuation suppression control section in FIG. 4, a method, in which the drive mode is detected by a polarity of a power factor of the inverter output (Vout, Iout) and the polarity of the bias voltage is changed by that detection result, has been known (see Non-Patent Document 1).
Further, a method, in which a value is obtained by multiplying a bias value calculated from the difference between the capacitor voltages VC1, VC2 by an even order wave of output frequency of the inverter and this value is added to a voltage command value V* (VU*, VV*, VW*), has been disclosed (see e.g. Patent Document 2). In this method, it becomes unnecessary to detect the polarity of the power factor.